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Full text of "Text Book Of Mechanical Engineering"

484

Screw Cutting*

Ex-ample 42.—Arrange the gearing of a single purchase crab so
that 6olbs. on a 15" handle may raise half a ton from a barrel 10"
dia., neglecting friction. (Eng. Ex., 1891.)

Mech. adv.

W       follower

_ .         x

handle

driver       barrel rad.

1120

~6o~

1 1 20     follower

_ ___

60

1 5

_            _ ^

driver        5

follower   __   6*22
driver    ~~~     I

So the pitch line diameters may be 6" and 37'32" for pinion and
wheel respectively, as in Fig. 442. (Seep. 955.)

Example 43.—A shaft A has a spur wheel of 120 teeth, which drives
a pinion B with n teeth. On shaft B is a wheel of 132 teeth driving
a pinion C of 10 teeth. Lastly, on shaft C is a wheel of 48 teeth
driving a pinion D of 8 teeth. A turns at 2 revs, per m. Find speed
of D. (Eng. Ex., 1885.)

The wheels are shewn in Fig. 443.

vel. D  __ followers = 120>jx * 32^*^48 __ 864
vel. A ~" ""drivers           11 x 10 x 8           I

/. D makes 864 x 2 = 1728 revs, per m.

Example 44.—Two men at a crab exert 60 Ibs. each on a 16"
handle. The pinion has 12 teeth, the wheel 72 teeth, and the chain
barrel is 12" diameter. Find the load raised, neglecting friction.
(Eng. Ex., 1888.)

followers x handle = 72 x 16 _  16

i

W
Mech. adv. —

W  :

drivers x barrel rod.
16 x P = 16 x 120 =

12x6
1920 Ibs.

. 955.)

Example 45.—In a Weston block the diameter of the large sheave
i s 10", and that of the smaller 9". Find the load raised by a pull of
50 Ibs., neglecting friction.

W       2 A       2 x 10     20
Mech. adv. -fr — 7—u == —;— ^ —

JT            _rL — JD                 I                   I

/. W « P x 20 = 1000 Ibs.

(See Appendix 7K, /. 954, without fail.}

Change Wheels in Screw-cutting.—-General principles
-re explained at pp. 147 and 212, it being shewn that: